OBJECTIVES: Previous experiments have revealed that nuclei of eukaryotic cells contain an enzyme which polymerizes the ADP-ribose moiety of NAD through a 1', 2' ribose-ribose linkage to form poly(ADP-Ribose). The polymer is at least in part associated with basic chromosomal proteins. Synthesis of poly(ADP-Ribose) occurs predominantly at the S/G2 transition point during the cell cycle, and cell divison does not proceed when polymer synthesis is blocked. The current objective is to characterize the nuclear proteins to which poly(ADP-Ribose) is attached and to elucidate the significance of the attachment of polymer to proteins insofar as cell division is concerned.